Protein kinase F regulates the virulence of Mycobacterium tuberculosis

The serine/threonine protein kinase F (PknF) of Mycobacterium tuberculosis (Mtb) has poorly defined targets and functions but is involved in limiting NLRP3 inflammasome activation in murine macrophages and dendritic cells in vitro. The importance of PknF for the virulence of Mtb in vivo is not known. Here, we demonstrate that the Mtb CDC1551 deletion mutant of pknF (∆pknF) expresses significantly increased levels of the lipid pthiocerol dimycoserosate (PDIM), a polyketide lipid with pro-virulence properties. The ∆pknF mutant strain compared with the Mtb and complemented strains showed a 100-fold increase in growth at day 28 and about a 10-fold increase in growth at days 90-98 in the lungs of mice. The increase in pulmonary bacterial loads after infection with the ∆pknF strain was conserved even in Nlrp3-deficient mice, arguing that PknF modulates Mtb virulence independently of NLRP3-inflammasome activation in mice. Staining of lung sections revealed increased inflammation in the lungs of ∆pknF strain-infected mice when compared with Mtb and the complemented mutant strain. Highly susceptible B6.Sst1^S mice displayed decreased host resistance, with significantly decreased survival when infected with the ∆pknF strain compared to the complemented strain or Mtb. In conclusion, our data suggest that expression of PknF, as a modulator of multiple downstream effector proteins, restricts the virulence of Mtb in the lungs of mice through an NLRP3 inflammasome-independent mechanism, but potentially via suppressing production of the virulence lipid, PDIM.